Field of the Invention
This invention relates to an acrylonitrile/butadiene/styrene (ABS) resin composition having improved molding properties suitable for use in the rotational molding method that is currently receiving much commerical attention.
Recently, rotational molding, also called rotocasting, has attracted special interest as a practical molding method for manufacture of articles, particularly those made from polyethylene resins. This is in keeping with a trend of development of rotocasing as an economical and advantageous method of molding large-sized articles.
Rotocasting is generally applicable to the molding of thermoplastic resins, and is applicable to plastic materials in the form of power, grains or liquid. Commercial rotocasting devices are recommended by the manufacturers as applicable to the molding of ABS resin as well as polyethylene, polyvinyl chloride, polystyrene, nylon, cellulose butyrate and cellulose acetate.
However, the rotocasting method has a number of disadvantages, many of which apply particularly to the use of ABS resins.
Thus, resin powders prepared by conventional emulsion polymerization are too small in particle size for use in rotocasting. Hence such powders are usually heated to melt them, and are then formed into pellets, the size of which is regulated to be from about minus 10 to plus 100 mesh size. If granules of significantly larger size are used in the rotocasting process or if granules of significantly intricate shape are used, a part of the granules remain unmolten in the process, and a large mass is formed in the machine, resulting in a rough surface or an uneven thickness of the finished product. Also, when the process involves pouring the resin into small spaces such as the space between double walls, bridges tend to be formed making the molded product defective or otherwise unsatisfactory. Further, by comparison with products made of the same resin by other known resin molding methods such as injection, extrusion or blow molding, the products conventionally obtainable by rotocasting have significantly inferior mechanical strength.
ABS resin is known to possess such disadvantages as the foregoing. Heretofore the molding properties of this resin, when used in rotocasting, have been controlled mainly by regulating the particle size and particle shape, and partially by regulating the melt viscosity at the time of fusion of the composition per se. As yet these methods have not provided a fully satisfactory molding process for ABS resins. Conventionally the known shortcomings of the inferior molding properties of ABS resin have been partly compensated for by using molding techniques such as the use of a foaming agent, a technique which is not affected by the inherently poor rotational molding property of ABS resin.